In general, this invention relates to a method and apparatus for accumulating certain data relevant to the operating history of a turbomachine and, in particular, this invention is especially applicable to data relating to "incidents" occurring at a turbomachine rotor bore.
The art of rotor forging or the formation of large diameter rotors for turbomachines of the type used in the generation of electrical power may result in the inclusion of stress risers or stress points in the vicinity of the centerline axis of the rotor. These built-in stress risers and stress points are inclusions and impurities which occur in the rotor at the time it is formed. These impurities, if left in place, could propagate or enlarge and may ultimately lead to reduced operating flexibility or an early replacement of a rotor into a turbomachine. It is therefore desirable to omit these imperfections at the rotor core and hence, it has become the practice of turbomachine manufacturers to bore out the rotor core so that a rotor bore hole several inches in diameter is formed at the rotor centerline. The rotor bore hole therefore obviates much of the undesirable material in the rotor while also providing a convenient site for rotor ultrasonic testing.
Modern rotor materials for steam turbine rotors are comprised of steel alloys which from a bore stress analysis point of view may have the following characteristics. At lower bore temperatures the rotor material exhibits brittle characteristics, whereas at higher bore temperatures the rotor material exhibits ductile characteristics. The boundary between the lower (brittle) and the higher (ductile) temperatures is the so called Fracture Appearance Transition Temperature (FATT) which may occur in modern rotor materials somewhere in the range of from 200.degree. to 350.degree. F. The Fracture Appearance Transition Temperature or transition temperature may be used to delineate certain temperature-stress regions according to one aspect of the invention. At temperatures below the transition temperature where the rotor material may exhibit more brittle characteristics the design concern is with rotor burst whereas at temperatures above the transition temperature the design concern is more directed toward crack elongation. According to the invention, certain stress values occurring below the transition temperature may be considered undesirable whereas those same stress values above the transition temperature may be considered acceptable. Hence, the transition temperature divides a stress versus temperature plot into brittle and ductile regions which thereafter may be subdivided in accordance with this invention into zones of potential risk.
One object of the invention is to determine whether the instantaneous bore temperature has exceeded the transition or FATT temperature in order to determine which allowable stress values (brittle or ductile) will apply.
Another object of the invention is to compare instantaneous bore stress values to allowable bore stress values to determine the occurrence of "incidents" wherein the actual calculated bore stress exceeds the allowable bore stress value.
Finally, it is a further object of the present invention to accumulate data relative to "incidents" in registers representative of the brittle side of FATT and the ductile side of FATT in order to determine the operating history of the rotor. The data can then be analyzed to determine the manner in which the turbomachine is being operated and the information can be further utilized as a basis for analyzing rotor condition without actually shutting down and inspecting the turbomachine. Further, the data is useful in planning a maintenance schedule for the turbomachine.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description taken in connection with the drawings.